In recent years, purification of water using a photocatalyst has been considered. The purification of water with the photocatalyst is a very safe purification method from the viewpoint of nonuse of hazardous substances to human bodies, such as chlorine and ozone. In such the use, a photocatalyst film (comprising a base material coated with a photocatalyst) is considered in general. In this case, various adherents may fix onto the photocatalyst film and cause blockage of light and coating over the surface, which prevent the photocatalytic performance. Many of the adherents are resulted from metal ions present in water, which are adsorbed onto the surface of photocatalyst fibers and oxidized. The use of a fiber-shaped photocatalyst described in Patent Document 1 causes an excellent expectable purification performance but allows adherents to easily fix thereon as a problem because of a larger surface area than the photocatalyst film. Namely, if the fiber-shaped photocatalyst can be prevented from adherent fixation, it is very useful as a photocatalyst for processing water.
For removal of such the metal oxides attached onto the photocatalyst, processes of acid cleaning and cutting off may be applied. These processes are dangerous, though, and may possibly deteriorate the photocatalytic performance, and accordingly different measures are required. For example, there is a process for applying activated oxygen to precipitate metal ions in processed water and filtering the produced solid as described in Patent Document 2. There is another process for using a potential having the same polarity as the polarity of metal ions, which is applied on the photocatalyst surface.
Patent Document 1: JP Patent No. 3465699
Patent Document 2: JP 2004-181409A
Patent Document 3: JP 2003-285069A